1. Field of the Invention
The present invention relates to a controlled-release (also referred to as slow-release, sometimes) granular coated fertilizer, and a method for producing the same. More particularly, the present invention relates to a controlled-release granular coated fertilizer, which is coated with a thermosetting resin such as urethane resin, epoxy resin or the like, and a method for producing the same.
2. Description of the Related Art
A so-called slow-release granular fertilizer wherein a dissolution rate of the fertilizer is controlled by coating or encapsulizing a granular fertilizer with a resin, sulfur, etc. has been used for years. For example, Japanese Patent Kokoku Publication Nos. 40-28927, 44-28457, 37-15382 and 42-13681 disclose various coating materials and coating methods.
U.S. Pat. No. 3,264,089 discloses a coated fertilizer wherein a coating material is a urethane resin made of a reaction product of a polyisocyanate compound and a polyol compound, while U.S. Pat. No. 3,264,088 discloses an epoxy resin coating material. With respect to a method for producing a coated fertilizer, comprising using a urethane resin as a coating material, Japanese Patent Kohyo Publication No. 1-500661 discloses a technique characterized in that an excessive amount of an isocyanate compound is reacted with a urea fertilizer on the surface to form a base coat and a polyol compound is reacted with excess isocyanate groups thereon. Japanese Patent Kohyo Publication No. 7-500560 discloses a technique characterized in that a mixed resin of a polyisocyanate compound and a polyol compound is coated to form each layer having a thickness of 10 to 30 .mu.m which is then cured by treating with an amine mist.
However, these techniques have industrially disadvantageous problems because the object of realizing less coating defects and controlled dissolution of the fertilizer is not sufficiently satisfied or a complicated apparatus and complicated steps are required in the production.
In these methods, a dissolution rate of the fertilizer to the external environment is decided by the property of the resin or the kind of the fertilizer. Therefore, in order to retard the dissolution rate, it is effective to increase a coating film thickness of the resin. However, since a proportion of an effective fertilizer per unit weight of the coated fertilizer is decreased, it is necessary to apply a large amount of a coated fertilizer to soil. A large amount of the resin is required and, therefore, there arise a problem about cost and a problem that the productivity is lowered by prolongation of the time required to produce the coated fertilizer.
On the other hand, a technique of coating a fertilizer with a hydrophobic compound such as wax has been known, and Japanese Patent Kokoku Publication No. 6-2632 and Japanese Patent Kokai (laid-open) Publication No. 52-93561 disclose a method of wax coating to prevent hygroscopic fertilizer products from adhering each other.
In the coated fertilizer, which is coated with a resin, a problem of adhering of resins does not occur. Therefore, it is not necessary to add a hydrophobic compound.
An object of the present invention is to provide a granular coated fertilizer wherein a dissolution rate is further retarded than a slow-release granular coated fertilizer, which is coated with a thermosetting resin, and a method for producing the same.